Parison lubrication apparatus

ABSTRACT

An apparatus for enhancing the uniformity of wall thickness of an article having a high aspect ratio horizontal cross section and produced from a parison in a blow molding process is disclosed. A barrier layer is applied to a selected area of the outer surface of each parison prior to its introduction into the blow molding apparatus, the barrier layer being effective to prevent premature freezing of the blown parison upon contact with a wall of the blow molding apparatus. The barrier layer application apparatus includes a generally contoured pad coupled to a supply of the barrier material, the pad being situated to contact a selected portion of each parison prior to its introduction into the blow molding apparatus.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 07/284,171filed Dec. 14, 1988, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for the lubrication of a parisonfor use in a blow molding process. Utilization of the present inventionpermits blow molding of containers having elliptical horizontal crosssections that have a high aspect ratio.

The production of containers through the use of a blow molding processis widely practiced in the container manufacturing industry. The firststep in this process consists of extruding or, preferably, injecting amolten substance into a mold cavity. Upon hardening, a thick walledhollow object commonly known as a preform or a parison is produced. Thisparison is then brought to a temperature conducive to blow molding andplaced in a blow mold. A high pressure gas or mixture of gasses such asair is then injected into the interior of the parison, causing theparison to rapidly expand until it contacts the walls of the blow moldcavity. The final form of the container is produced by the walls of theblow mold cavity cooling the substance and preventing further expansionof the substance.

Many substances can be used in a blow molding process, but glass andthermoplastics are most commonly used in the container manufacturingindustry. High density polyethylene, polypropylene, and polyvinylchloride based plastics are all suitable for use in a blow moldingprocess. For some thermoplastics, such polyethylene terepthalate, anadditional step of biaxially orienting the thermoplastic by stretchingthe parison can provide superior clarity, drop resistance, and tensilestrength to the finished product.

However, blow molding is not an ideal method for producing certain typesof finished articles that have shapes whose cross section is notsubstantially circular. Containers having elliptical cross sections ofhigh aspect ratio are difficult to produce using this method because theparison walls freeze against the mold walls upon contact, stopping anyflow of material from the frozen areas of the parison to those areas ofthe parison that are still expanding due to air pressure. As a result,the walls of the container are abnormally thick in those areas thatfirst contact the mold near the minor axis of the mold, and abnormallythin in those areas near the major axis of the mold.

A further problem is the migration of the crystallized plastic spruepoint when thermoplastics such as polyethylene terepthalate are used.When the parison is injection molded, the plastic nearest the injectionsprue is abnormally hot compared to the rest of the plastic. The heat inthe sprue area causes polyethylene terepthalate to crystallize into amore brittle form. This crystallized plastic, sometimes known as a sprueartifact, constitutes an unavoidable weak point in the finished product.However, the possibility of failure at the point of crystallizationrepresented by the sprue artifact can be minimized by appropriate designof the finished product. If the sprue artifact is centered at the bottomof the finished product, and protected from direct contact with otherobjects by raised ridges or feet, the possibility of failure due toimpact is significantly reduced.

When products having a substantially circular cross section are blowmolded, the sprue artifact remains generally centered in a protectedposition because the blown parison nearly simultaneously contacts thewalls of the blow mold cavity. However, if the mold has a high aspectratio elliptical horizontal cross section, the sprue artifact canmigrate to areas outside the protective structures due to non-uniformthinning caused by freezing of the plastic to the mold walls. If thesprue artifact migrates outside of the protected area, the finishedproduct is useless because of the high risk of stress induced containerfailure at the sprue artifact point.

Several methods have unsuccessfully been attempted to remedy the problemof wall thinning and migration of the sprue artifact. Varying blowpressures, changing mold temperatures and changing mold profiles havefailed to alleviate the problems associated with blow molding containershaving a high aspect ratio elliptical, horizontal cross sections. Moresubstantial changes, such as forming a blow mold from nickel impregnatedwith a slippery fluorocarbon compound such as Teflon have also failed toprevent wall thinning due to premature freezing contact with the moldwalls. Other alternatives such as designing preforms with ellipticalshapes could work in certain situations, but the cost of retooling tohandle and properly orient variant shapes can be prohibitive, especiallywhen low volume or marginally profitable production runs arecontemplated.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for preventing the prematurefreezing contact of a blown parison with the walls of the mold cavity,thereby reducing the differences in wall thickness and sprue artifactmigration caused by the freezing contact. This is accomplished by theaddition of a barrier layer between the walls of a mold cavity and aparison. The barrier layer prevents the premature cooling and consequentfreezing of the parison material to the walls of the mold cavity, andpermits the continued flow of material toward the expanding sections ofa blown parison.

Many barrier materials are suitable for use in this invention. A barriermaterial should not significantly interact chemically or physically witheither the parison material or the walls of the mold cavity, shouldremain stable under the temperatures typically encountered during theblow molding process, and should not present a hazard to human health.Preferred embodiments of a barrier material for the present inventionare lubricants capable of being applied as a surficial liquid to thewall of the parison. A most preferred embodiment is a liquid such asAP-5, certified for use as a lubricant by the FDA.

A lubricant acting as a barrier material can be applied in a number ofways. One suitable embodiment for application of a lubricant is realizedby an apparatus for dipping a suspended parison into a bath of thelubricant barrier material. The wetted parison is then transferred tothe proper position in the blow mold for subsequent processing. A driptray or drip wiping pad can be provided to ensure that excess lubricantis returned to the lubricant bath, assuring economical use of thelubricant barrier material and preventing spread of the lubricant fluidto inappropriate areas.

An alternative embodiment contemplates the use of spray devices toevenly disperse the lubricant barrier material on the surface of aparison. The region about the spray area should have suitable protectivebarriers, such as walls, fume hoods, or forced drafts in order toprevent unwanted dispersion of the lubricant. The use of sprayapplication is not recommended however for those applications involvingcontainers for foods or beverages. The problem of contamination of theinterior of the parison by airborne spray particles of the lubricantmakes sanitary use of spray devices problematical unless the applicationtakes place while the mouth of the parison is closed such as by aparison handling device.

A preferred embodiment of the invention that can be used with virtuallyany blow molding machine without expensive modifications to the blowmold cavity area, and with little danger of contamination utilizes acontoured application pad. An application pad, composed of a materialsuitable for applying a liquid coating of the lubricant to a parison, isconstructed so as to conform to the desired areas of the parison thecontoured application pad can be constructed in either one piece or aplurality of separate pieces. The application pad can also have either afixed structure or be rotatable.

Means for supplying the lubricant barrier material to the applicationpad are also necessary. For limited use, this can be as simple aspouring the lubricant barrier material directly on the applicant padbefore use. For long term applications, an automatic system of supplyinglubricant barrier material to the application pad is recommended, suchas using a rotatable application pad that at some point during rotationencounters a bath of the liquid. Such a rotatable application pad can berotatably driven either by frictive contact with parisons laterallymoving across the edges of the application pad, or by a drive mechanismthat causes rotation of the application pad. For continuous use, theamount of lubricant barrier material applied to the parison should notexceed the amount of lubricant barrier material supplied to theapplication pad.

It is accordingly an object of the present invention to provide anapparatus for the application of a barrier material that prevents thepremature freezing of a blown parison to the walls of a blow moldcavity.

It is a further object of this invention to provide an apparatus for theblow molding of objects having high aspect ratio elliptical horizontalcross section.

Another object of this invention is to provide an apparatus for theprevention of sprue migration in blown plastic parisons.

It is also an object of this invention to provide an apparatus forensuring that a blown container has substantially uniform wallthickness, with no abnormal thinning or thickening of the walls.

Another object of this invention is to provide an apparatus for applyinga lubricant that acts as a barrier material between a parison and thewalls of a blow mold.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the invention will become apparent tothose skilled in the art upon consideration of the following detaileddescriptions of preferred embodiments exemplifying the best mode ofcarrying out the invention as presently perceived. The detaileddescription particularly refers to the accompanying figures in which:

FIG. 1 is a view of a blown container and a dotted line indicating thesize and position of a parison before the container is blown;

FIG. 2 is a horizontal cross sectional view of a blow mold showing theoriginal placement of the parison and two representative positions ofthe parison walls during expansion;

FIG. 3 is a partial cross sectional view of a fixed application padcontoured to fit a parison transported in contact with the applicationpad;

FIG. 4 is a view of a contoured, mechanically driven rotary applicationpad;

FIG. 5 is a view of a rotary application pad, friction driven by thetransport of parisons attached to a separate conveyor;

FIG. 6 is a side view of a friction driven rotary application pad; and

FIG. 7 is a view of a dipping application bath.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A side view of a blown container 12 is shown in FIG. 1. The size andposition of a parison 10 in relation to the blown container 12 isindicated by the dotted line. The shape of the blown container 12 issimilar to the shape of containers commonly used in the food industry tocontain salad dressings of various kinds. The upper region of the blowncontainer 12 has screw threading 13 suited for accepting a correspondingthreaded cap (not shown) in order to seal the container. A horizontalcross section taken in the region about screw threading 13, such asindicated by line a-b, would reveal a substantially circular wallstructure of the blown container 12 in that area. A horizontal crosssection taken in the lower regions of the blown container 12, such asindicated by line c-d, would reveal a substantially elliptical wallstructure having a high aspect ratio. The particular shape of theillustrated container 12 is not intended to limit the invention, butrather is intended merely to illustrate an example of a container havingahigh aspect ratio cross section.

For the purposes of this invention, aspect ratio is defined as theresultant of the division of the length of the major axis by the lengthofthe minor axis of a container. The container shape does not have toexactlycorrespond to a rigorously defined geometrical ellipse, but canbe any closed figure. For example, as those skilled in the art canappreciate, a substantially rectangular figure has a well defined aspectratio equal to the length of the rectangle divided by its width. Asimilar aspect ratio value can be derived for irregular or varyingcontainer cross sectional shapes by determining the average minimal andmaximal lengths of the crosssection of the container. A blown container12 has a low aspect ratio if the aspect ratio is between about 1.0 to1.1. A low aspect ratio ellipse may be considered to be substantiallycircular. A blown container 12 has amoderate aspect ratio if it has aratio of major axis length to minor axis length of between about 1.1 to1.7. A high aspect ratio blown container 12has an aspect ratio more thanabout 1.7. By utilization of the term "aspectratio" no limitation is seton the variety of shapes of containers that canbe injection blow moldedby the use of this invention.

The blow molding process may be understood by referring to FIG. 2, whichshows a cross section of a blow mold 14 used to form the blown container12 shown in FIG. 1. The horizontal cross section of the blow mold 14shownin FIG. 2 is the portion that forms the corresponding part of blowncontainer 12 indicated by line c-d. FIG. 2 also shows a cross sectionalview of parison 10 in a mold cavity 15 that corresponds to the parison10 indicated by the dotted lines in FIG. 1.

The blow molding process involves the fixed placement of the parison 10in the mold cavity 15 formed by the blow mold 14. High pressure air isinjected into the interior of parison 10, causing the parison to expand.The wall of parison 10 thins during this expansion, as can be seen bynoting the relative wall thicknesses of parison 10 before expansion, andthe parison at later times, as illustrated by parison 10a and parison10b.The expansion is ideally uniform so that the wall thickness of theend product, the blown container 14, is also uniform. If the wallthickness isnot substantially uniform, the blown container ispotentially useless because of the high probability of containmentfailure in any thin wall sections. Even if adequate wall thickness ismaintained, a non-uniformity of wall thickness could entail unsightlyoptical distortions that would render the blown container 14 unsuitablefor use.

A common cause of non-uniformity in wall thickness of high aspect ratiocontainers such as blown container 12 can be attributed to the freezingcontact of the expanding plastic material of parison 10 with the moldwall16. Upon direct contact with the mold wall 16 the plastic materialof parison 10a freezes, changing from a semi-fluid plastic state to aglassy or partially crystallized state. That portion of the materialwhich is frozen is effectively removed from the amount of materialavailable to continue expansion, since only non-frozen plastic materialis capable of expansion. As the parison 10a continues to expand due tointernal air pressure, still more material is frozen by direct contactwith the mold wall 16, further diminishing the amount of stillsemi-fluid plastic material that can expand in the mold cavity 15. Theend result is a blown container 12 that has abnormally thick walls inthe areas near the intersection of the minor axis and the wall of theblown container 12, which came in first contact with the mold wall 16,and abnormally thin walls near the intersection of major axis and thewall of the blown container 12.

The present invention alleviates the problem of wall thinning due tofreezing of the material comprising parison 10 by the placement of abarrier layer (not shown in FIG. 2) between the parison 10 and the moldwall 16. A preferred embodiment of a barrier layer is a liquid lubricantthat does not significantly react either physically or chemically witheither the material composing parison 10 or the mold wall 16. A morepreferred embodiment is a liquid lubricant safe for use with foods orbeverages, such as silicon lubricants or edible oils. A most preferredembodiment of a barrier material is a liquid lubricant such as AP 5,whichdoes not leave a sticky residue that could necessitate periodiccleaning ofthe blow mold 14.

Various methods can be used to coat desired areas of the parison 10 witha lubricant prior to placement of the parison 10 in the blow mold 14. Asimple method contemplates the use of a wicking application pad 18,contoured to match the surface profile of the areas of a parison 10which are to be coated with a lubricant 22 for use as a barriermaterial. A conveyor system laterally transports the parison 10 so thatcontact with the contoured areas of wicking application pad 18 ismomentarily maintained, permitting the transfer of lubricant 22 topredetermined areasof parison 10. The wicking application pad 18 isconstructed of an absorbent material that has a wicking uptake oflubricant 22 contained in lubricant container 20 sufficient to ensure acontinual supply of lubricant 22 to the contoured surface of applicationpad 18.

An alternative embodiment for application of a lubricant to a parison 10involves the use of a rotating application pad 28 as illustrated in FIG.4. The rotating application pad 28 is contoured to match the parison 10surface in the desired areas for coating with a lubricant 22. A rotatingdrive means 26 is used to cause rotation of the rotating application pad28. The rotating application pad 28 is supported for rotation by anapplication pad support 30 acting as an axle means for the rotatingapplication pad 28. A rotating drive 32, which can be an electric motororother device capable of imparting a rotary force, causes rotation of adrive pulley 34. This rotary motion is communicated by means of a drivebelt 36 to the rotating application pad 28. The revolution speed ofrotating drive 32 can be determined so that partial, single or multiplerevolutions of the rotating application pad 28 occur when the parison 10is in contact during its lateral transport across the contoured surfaceofthe rotating application pad 28. In operation, the rotatingapplication pad28 is partially immersed in a bath of lubricant 22.

Alternative embodiments of a rotating drive are also contemplated. Forexample, direct mounting of the rotating application pad 28 on a drivepulley 34 without an intermediary drive pulley would also ensurerotation of the rotating application pad 28. Other means of causingrotation of a rotating application pad 28 can be readily envisaged bythose skilled in the art.

The use of a rotating drive means 26 is not necessary for the operationof this invention. FIG. 5 illustrates a rotating application pad 28 thatis not driven by a rotating drive means 26. The rotating application pad28 is axially supported by an application pad support 30 that permitsfree rotation of the rotating application pad 28. The lower portion ofthe rotating application pad 28 is immersed in a bath of lubricant 22.The frictional forces generated by the substantially tangentialtransport of aparison 10 by parison conveyor 24 act to rotate therotating application pad 28 in such a manner that a continuous supply oflubricant 22 is transferred to application pad 28, and from there thelubricant 22 is applied to parison 10. A side view of a rotatableapplication pad 28 that is not driven is illustrated in FIG. 6. The useof a conveyor belt to tangentially move multiple parisons across thecontoured surface of the rotatable application pad 28 is shown.

However, application pads are not a necessary part of this invention.Alternative means of applying a barrier material such as lubricant 22 toaparison are contemplated. Spray devices are well suited for high speedcontinuous coating purposes, such as are envisioned for operation of thepresent invention but pose significant problems in terms of control ofover spray as previously discussed. In the alternative, dipping methodssuch as that illustrated in FIG. 7 could suffice to coat the desiredareasof a parison 10 with a lubricant 22. In FIG. 7, a parison conveyor24 having a dipping section 32, dips a parison into a bath of thelubricant 22 before transport of the parison 10 to a blow mold (notshown).

It is contemplated that the previously described apparatus will havediverse embodiments adapted for particular uses or environments. Theparticular embodiments previously described are not intended to limitthe scope of the invention, and it is intended that the following claimswill encompass alternative and equivalent embodiments of the invention.

I claim as my invention:
 1. The combination of a blow mold, a liquidlubricant and an apparatus for applying a coating of the lubricant to aparison for use in said blow mold comprising:a source of liquidlubricant, at least one application pad, means for supplying the liquidlubricant to the application pad, means for contacting an outsideportion of the parison with the at least one application pad, therebytransfering the liquid lubricant to form a coating on the outsideportion of the parison, and means for transferring the thus coatedparison into said blow mold while the lubricant coating is still in aliquid state.
 2. The combination of claim 1 wherein the at least oneapplication pad is rotatable.
 3. The combination of claim 2 furthercomprising a rotating drive means for rotating the at least oneapplication pad.
 4. The combination of claim 1 wherein the at least oneapplication pad is contoured to allow application of the lubricantcoating to selected areas of the parison.
 5. The combination of claim 4,wherein the at least one application pad is rotatable.
 6. Thecombination of claim 1 wherein said means for contacting the parisoncomprises a parison transport means for transporting the parisonadjacent to and in contact with said at least one application pad. 7.The combination of claim 6 wherein the at least one application pad isrotatable.
 8. The combination of claim 1 wherein said means forsupplying the lubricant to the application pad comprises a lubricantsupply bath containing the lubricant and a rotatable application pad atleast partially immersed in said lubricant.
 9. The combination of a blowmold, a liquid lubricant and an apparatus for applying a coating of thelubricant to a parison for use in said blow mold which has a horizontalcross section that has a high aspect ratio comprising:a rotatableapplication pad having a rotation axis, and a contoured surfacesymmetric about the axis; means for supplying a liquid lubricant to therotatable application pad comprising a lubricant supply bath containingthe lubricant into which the application pad is at least partiallyimmersed; a parison transport means for transporting the parison so asto come in contact with the contoured surface of the application pad toallow application of a coating of the lubricant to selected areas of theoutside of the parison; and means for transfering the thus coatedparison into said blow mold while the lubricant coating is still in aliquid state.
 10. The combination of claim 9 further comprising rotatingdrive means for rotating the rotatable application pad.
 11. Thecombination of a blow mold having a blow mold cavity, liquid barriermaterial for preventing premature freezing contact between a blownparison and the walls of said blow mold cavity, and apparatus forapplying the liquid barrier material to a parison comprising:applicatormeans for applying a liquid barrier material to only a selected outerportion of a parison prior to the introduction of the parison into saidblow mold cavity; reservoir means coupled to the applicator means andcontaining a supply of the liquid barrier material for supplying thebarrier material to the applicator means in sufficient quantity to coatthe selected portion of the parison; and transfer means for transferingthe thus coated parison into said blow mold cavity while the barriermaterial coating is still in a liquid state.
 12. The combination ofclaim 11 wherein the barrier material in the reservoir means comprises alubricant.
 13. The combination of claim 11 wherein the applicator meanscomprises a rotatable application pad having a rotation axis and acontoured surface symmetric about the axis, a portion of the contouredsurface being immersed in the supply of the liquid barrier material inthe reservoir means.
 14. The combination of claim 13 further comprisingrotating drive means for rotating the rotatable application pad.